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Carbonate, Ordovician, Peat moss, Sphagnopsida, Sphagnum, Vegetative microfossil


Premise of research. Climatically favorable conditions correspond with fossil evidence for dramatic Ordovician marine biodiversification, but coeval terrestrial biodiversity is less well understood. Although diverse Middle and Late Ordovician microfossils are interpreted as reproductive remains of early bryophyte-like land plants (consistent with molecular data indicating pre-Ordovician embryophyte origin), the vegetative structure of Ordovician plants remains mysterious, as do relationships to modern groups. Because distinctive fungal microfossils indicating land plant presence were previously reported from Ordovician carbonate deposits in Wisconsin, we examined another nearby outcrop for additional evidence of terrestrial biodiversification.

Methodology. Replicate collections were made from well-understood 455–454 Ma Platteville Formation carbonates of relatively low porosity and hydraulic conductivity. We employed measures to avoid contamination, and organic remains extracted by acid maceration were characterized by light and scanning electron microscopy and energy-dispersive X-ray spectroscopy.

Pivotal results. Multicellular organic fragments displayed distinctive cellular features shared with modern vegetative peat mosses but differed from modern materials, e.g., fossil presence of mineral coatings, absence of epibionts. Biometric features of mosslike microfossils isolated from carbonates collected and macerated 12 yr apart by separate investigators did not differ. Putative peat moss remains occurred with foraminifera similar in frequency and thermal maturity to types previously described from the same formation. No diatoms, pollen, or other indicators of post-Ordovician environments were observed.

Conclusions. The peat moss–like fragments described here are the oldest-known vegetative remains of land plants and the oldest fossils having distinctive features linking them to a modern plant group. These findings are consistent with peat moss recalcitrance properties that foster fossilization and molecular evidence that the peat moss lineage is 460–607 Ma of age. The new findings suggest that moss-dominated peatlands—recognized for globally significant roles in modern terrestrial biodiversity and C and N cycling—were present hundreds of millions of years earlier than previously thought.

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International Journal of Plant Sciences





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University of Chicago Press Journals

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DOI: 10.1086/686242